The celebration of mass on Christmas Eve is often given various origins. There are three that are most common.
The first comes from a legend according to which a cockerel was the first animal in the stable to announce the birth of Jesus with its crowing, so everyone knew.
Another says that in Jerusalem the custom was to have midnight mass followed by a procession to the most important temple in the city, where they celebrated a final mass, coinciding with the cockerel’s crow.
Yet another goes back to 5th century Rome, where Pope Sixtus III established the habit of holding a vigil at midnight on Christmas “after the cockerel crows”. This came from the fact the Romans called the start of the day the “cockcrow”, an expression also inherited in English.
Whatever the origin, as it’s a festive celebration, it is also traditional to offer the congregation hot chocolate or mulled wine and pastries.